Piezoelectric materials

ABSTRACT

PIEZOELECTRIC MATERIALS HAVING A COMPOSITION WHICH MAY BE EXPRESSED BY THE GENERAL FORMULA:   (1-X)PBTIO3$PB(ME1/2TE1/2)O3   (WHERE ME REPRESENTS AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MG AND ZN AND X DENOTES A VALUE OF 0.02 TO 0.25).

of.1o,'1'972 NOBORU .CHWOSEETAL v3,69'11430 PIEZOELECTRIC MATERIALS Filed June 3, 1971 TEMPERATURE (c) FIG. 2-

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TEMPERATURE (6) Patented Oct. 10, 1972 3,697,430 PIEZOELEC'I'RIC MATERIALS Noboru Ichinose, Harutoshi Egami, Katsunori Yokoyama, and Yohachi Yamashita, Yokohama, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-shi,

Japan Filed June 3, 1971, Ser. No. 149,613 Claims priority, appliicz/ltsiggigapan, June 13, 1970,

Int. c1. cob 35/46 U.S. Cl. 252-62.9 1 Claim ABSTRACT OF THE DISCLOSURE Piezoelectric materials having a composition which may be expressed by the general formula:

(1 x)PbTOaxPb(Me1/2Te1/2)O3 (where Me represents at least one metal selected from the group consisting of Mg and Zn and x denotes a value of 0.02 to 0.25).

high frequency region of the order of mHz.

As is well known, piezoelectric materials are used in a broad field of application including elements for oscillating ultrasonic waves, transducer elements of, for example, mechanical filters, elements of ceramic lters and ceramic resonators and elements of vibrometers and accelerometers. For such uses, there have been developed improved forms of piezoelectric materials of a binary metal oxide system of PbTiO3-PbZrO3. Such improvements have been attempted by incorporating additives such as Bi203, Cr2O3, MnOz or ZnO in said binary metal oxide system of PbTiOa-PbZrOa. There have also been proposed piezoelectric materials of a ternary metal oxide system of PbTiO3-PbZrO3-Pb(Mg1/3Nb2/3)O3. However, those prior art piezoelectric materials had a high dielectric constant and a Curie point of about 300 C., and were not useful when the service prevailed temperatures higher than said level. They had further drawbacks in that they had a dielectric constant as high as about 1000 and were not suitable for use high frequency regions.

It is accordingly an object of this invention to provide piezoelectric materials which have a suiciently low dielectric constant to be used as elements of high frequency filters and resonators.

Another object of the invention is to provide piezoelectric materials capable of operating under stable condition at high temperatures over 300 C.

According to this invention, there are provided piezoelectric materials having a composition which may be expressed by the general formula:

(where Me represents at least one metal selected from the group consisting of Mg and Zn and x denotes a value of 0.02 to 0.25).

The present invention can be more fully understood from the following detailed description when taken in conjunction with reference to the appended drawings, in which:

FIG. l is a curve diagram showing changes with temperature in the dielectric constant of two examples of this invention; and

FIG. 2 ils a curve diagram showing changes with temperature in the electro-mechanical coupling coeicient (hereinafter designated as K33) of the two examples of FIG. 1 and one reference sample of the prior art.

The piezoelectric materials of the invention are characterized by being prepared from a plurality of metal oxides of different valences compounded by solid phase reaction, that is, consisting of a binary system of perovskite structure: (l-x) PbTiO3-xPb(Me1/2Te1/2)O3 (where Me represents at least one metal selected from the group consisting of Mg and Zn and x denotes a value of 0.02 to 0.25).

The piezoelectric materials according to this invention may generally be easily manufactured by the known powder metallurgical process. For illustration, there are accurately weighed out the prescribed proportions of raw metal oxides such as PbO, Ti02, TeO, and MeO. They are thoroughly mixed in a ball mill or the like. The raw materials used may consist of other compounds which may be thermally converted to oxides, such as hydroxides, carbonates and oxalates of metals. The mixture is presintered at a temperature of, for example, about 600 Ito 900 C. and pulverized again in a ball mill or the like to a particle size of about 1 to 2 microns. To said powders is added water or polyvinyl alcohol as a binder. The mass is subjected to a pressure of 0.5 to 2 tons/ cm.2 and sintered at a temperature of 1000 to 1250 C. Since part of the PbO, one of the components, is likely to be evaporated OIT during said sintering, the operation is carried out in a closed furnace. The mass should be maintained at a maximum temperature for a period ranging between 0.5 and 3 hours. The sintered mass of metal oxides thus obtained is polarized by the known process, for example, by fitting a pair of electrodes to both sides of the sintered body and impressing across the electrodes a voltage having a D C. field intensity of 40 to 60 kv./cm. for one or two hours while in silicone oil at a temperature of to 200 C.

The reason why, in the piezoelectric materials of a binary metal oxide system according to this invention represented by the formula of (l-x) Mel/zTel/z) O3 x is so chosen as to have a value of 0.02 to 0.25, that is, the proportion of Pb(Me1/2Te1/2)O3 is limited to 2 to 25 mol percent follows: If x denotes a value of less than 0.02, the mass of compounded metal oxides will be more ditlicult to sinter and will fail to have the desired piezoelectric properties. If x increases over 0.25, the Curie point will be lowered (Tc will fall to below 400 C.), making it unsuitable under stable condition at higher temperatures than 300 C. In this case there will also arise a decline in the electro-mechanical coupling coecient KS3.

PbTiO3 has a Curie point approximating 500 C. and has heretofore been deemed as a hopeful component of piezoelectric materials. However, said oxide had no practical use because it raised a problem with sintering. In contrast, the piezoelectric materials of this invention include an additional component Pb(Me1/2Te1/2)O3, which concurrently acts as a mineralizer to facilitate the sintering of the piezoelectric system as a whole. This reduces the sintering temperature and suppresses the evaporation of PbO, one of the components, eventually enabling a compact piezoelectric product to be easily manufactured.

This invention will be more fully understood by reference to the examples which follow.

There were accurately weighed out the prescribed proportions of PbO, T102, Te03 and MeO so as to constitute 4 Ceramic resonators from piezoelectric materials having the same compositions as Examples 7, 10 and 18 were prepared. A determination was made of changes with time and temperature in the resonance frequency of said resa piezoelectric material of a binary metal oxide system onators. The results are reported in Table 2 below, showhaving a composition of ing that said samples had suciently good time and tern- (1 x)PbTi09XPb(MC1/2TC1/2)03 piiiture characteristics to be put to practical applica- (where Me represented Mg or Zn and x denoted a value TABLE 2 of 0.01 to 0.30): The metal oxides were thoroughly mixed in a ball mill, presintered at a temperature of 850 C., Ex-7 ELIO EL 18 and further pulverized again in a ball mill to a particle Tmperatur iietnigiegi )or reso/ttige@ 71 66 60 size of 1 to 2 microns. There were also prepared samples Chflxlgrcih time ion resoal'g'quec-- from the conventional compositions. Thus there were pro- (iii one year) pement +0.19 +0.25 +0.15 vided twenty-three samples of powdered metal oxides of l5 both this invention and the prior art. After adding poly- As seen from the foregoing samples, the piezoelectric vinyl` alcohol as a binder to these powdered samples, materials of this invention have many useful features. the mass was molded at a pressure of 1 ton/cm.2 and They are useful under stable condition at temperatures heated to 1000 to 1250 C. and maintained for one and over 300 C. and also in high frequency regions, and disa half hours at said temperature, obtaining sintered rods play excellent performance as various transducer ele- 1 mm. in diameter and 3 mm. long. The rod samples were ments due to minimal change in time and temperature measured for density, and dielectric properties by tting characteristics. electrodes thereto. After being polarized by being im- The piezoelectric metal oxide materials of this invenpressed one anda half hours with a voltage having D C. tion may be used, for example, in the following applicaeld intensity of kv./cm. in silicone oil at 190 C., 25 tions: thee piezoelectric properties of the mass were determined (1) Determination of vibration, acceleration and pres by the standard process set forth in, for example, the Prosure applied to high temperature objects approaching 500 ceedings of I.R.E., vol. 137, p. 1378-1395, 1949. Table 1 C. or other objects subjected to sharply changing temperabelow presents the results of determination, together with ture and also determination of pressure prevailing in the the compositions ofthe sintered samples. 30 interior of such high temperature objects.

TABLE 1 (i-npb'iios- IPb(Mei;Tei4)0s F.T. Kaz Samples Me z C.) D e (percent) Qm Tc C Reference:

0. 02 1, 240 7. 51 20s 0. 32 417 521 0. 02 1, 240 7. 53 185 0.34 35s 515 0.05 i, 200 7. es 271 0. 3s 503 503 0. 05 1, 200 7. 70 240 0. 37 407 500 0. 07 1, iso 7. 30 253 0. 42 512 434 0. 07 1, 130 7. s2 225 0. 44 584 431 gg 1, iso 7. s1 247 0 45 590 433 0I i0 1,150 7. ss 199 0.40 435 43s 0. 10 1, 150 7. 87 122 0. 42 435 450 0. 14 1, 130 7. 74 155 0. 3s 397 453 0. 14 1, 130 7. 77 160 0. 39 375 440 gg; 1,130 7.75 16s 0.40 391 450 0.18 1, 7. 70 159 0. 35 322 432 0.13 1,110 7. es 147 0. 3s 34s 43s 0.22 i 1,090 7.54 173 0.35 303 420 0. 22 1, 090 7. e0 153 o. 35 319 419 0. 25 1, 000 7. 55 141 0.31 295 403 0. 25 1, 050 7. 57 139 0.33 277 401 823g 1, 050 7.55 0.34 300 410 In Table 1 above, RT. denotes the sintering temperature (2) Application of supersonic waves to high C.), D density (at 23 C.), e the dielectric constant temperature objects (kHz., at 23 C.), K33 the electro-mechanical coupling 60 The piezoelectric materials of this invention can be flefletu .(mve melhmcqlit'vaaiear used as a source of supersonic waves in Working high temclangs wilf tilnih theedlieallectirclnstalnt e of Examples perature Objects using Supersonic Waves or as an element .n 1 and 9, the curves shown in FIG. 1 were obtained. The ,for examlm g said objects usmg supersonic Waves curve a represents Example 1 and the curve b, Exain- 65 (3) Generation of strong supersonic waves plc 9. When a determination-was made of changes with time in the electro-mechanical coupling coeiiicient K33 of Wh e11 Sublefted t? Vlgl'ous Vlbratlons O fdlnafy P1920- Said Samples, the curves of FIG 2 were obtainuh Due electric materials fail to be useful due to high heat buildto the high Curie point, Said samples exhibited an electro. up. However, those of the invenotion withstand application mechanical lcoupling coeicient KS3 which varied mini- 70 at a tmpel'aul'e'xceedmg 300 C., offering great advanmally over abroad temperature range of 200 to 400 telg@ 1 genefatmg Strong SuPefSOmC Waves by heavy C., proving that theyl would be available for use at the V1brat1n5 highest temperature ever allowed for any piezoelectric 4 A l.

i material. `Referring to FIG. 2, the curve c, denotes Expp cation m hlgh frequency reglons ample 1, the Ycurve d, Example 9, and the curve e, refer- 75 Conventional piezoelectric materials have the drawence 3.

back that they have a large dielectric constant e.g., 1000,

and are unadapted for use in high frequency regions. Generally, impedance Z is expressed by an equation (where d represents the thickness of a sample, S its cross sectional area, f the frequency used and e a dielectric constant). Therefore, d should be reduced in inverse proportion to f, so that the impedance may be eventually indicated as Z oc 1/ U21-S). If f increases, Z will sharply fall because f2 is a large factor. Though the matching of Z requires S or e to be reduced, it is more advantageous to decrease e, because S is subject to certain limitation from the standpoint of working. The piezoelectric materials of this invention have a dielectric constant e of about 150, that is, as low as to 3&0 of that of the prior art product. Accordingly, in the case where the prior art piezoelectric materials are available up to the frequency of 10 mHz., those of this invention permit application at a frequency as 50 mHz.

As mentioned above, it will be understood that the piezoelectric materials of the invention can be used in the applications which have been impossible with those of the prior art. A problem is caused when a high frequency filter or resonator changes frequency with time and temperature. Since, however, the present product varies minimally in frequency characteristics, it has several practical applications.

What is claimed is:

1. Piezoelectric metal oxide materials having a composition expressed by the general formula where Me represents at least one metal selected from the group consisting of Mg and Zn, and x denotes a value of 0.02 to 0.25.

References Cited UNITED STATES PATENTS 3,268,453 s/1966 ouchi eral. 252-629 3,309,168 3/1967 Bayer 252-629 x 3,463,732 8/1969 Banno etal.

U.S. Cl. X.R. 106-39 R 

